OpenAI: While not founded by Elon Musk, he was one of the original co-founders and served as the chairman of the board until 2018. OpenAI is a research company that aims to promote and develop friendly AI in a way that benefits humanity as a whole.
SolarCity: Founded in 2006 by Peter and Lyndon Rive (cousins of Elon Musk), SolarCity was a provider of solar energy services. Elon Musk was the chairman of the board and the largest shareholder of SolarCity before it was acquired by Tesla in 2016.
Elon Musk's involvement with these companies has varied from being a founder and CEO to a significant investor and advisor. His entrepreneurial ventures have made him one of the most recognized and influential figures in technology and space exploration.